Calculate free chlorine ppm from an ORP reading in mV using your pool’s pH, cyanuric acid and water temperature, or solve for the ORP or ppm you need.

The redox value shown by your ORP probe or controller.

pH decides how much of the sanitizer sits in its active acid form.

Enter 0 for indoor or unstabilized water. Stabilizer suppresses the ORP reading.

+ Advanced: temperature and probe calibration

Orp To Ppm Formula

The following formula can be used only when you have established an appropriate, application-specific linear calibration between ORP and a particular concentration (ppm) over a limited range (for example, via lab testing in the same water chemistry, pH, temperature, and sensor/reference setup). ORP itself is an electrochemical potential (mV) and does not have a universal conversion to ppm.

PPM = ORP / K

Variables:

  • PPM is the concentration (parts per million) of the specific substance you calibrated for
  • ORP is the oxidation-reduction potential (typically measured in mV versus a reference electrode)
  • K is the conversion factor (slope) from your calibration (mV/ppm)

To estimate the concentration in parts per million using this simplified model, divide the ORP by the calibrated conversion factor. Note: many real ORP-to-concentration relationships are not linear and may include an offset (intercept). If your calibration has an offset, you may need to subtract the baseline ORP (at 0 ppm) before using this formula.

Illustrative ORP (mV) to PPM Conversion Table (Example linear calibration: K = 300 mV/ppm)
ORP (mV) PPM
4001.333
4251.417
4501.500
4751.583
5001.667
5251.750
5501.833
5751.917
6002.000
6252.083
6502.167
6752.250
7002.333
7252.417
7502.500
7752.583
8002.667
8252.750
8502.833
8752.917
* Rounded to 3 decimals. Assumes linear model ORP = K × PPM with K = 300 mV/ppm as an example only. Actual ORP–PPM relationships depend on the redox chemistry, pH, temperature, and sensor/reference conditions.

What is Orp To Ppm?

ORP (oxidation-reduction potential, also called redox potential) is a millivolt (mV) measurement that indicates how strongly a solution tends to oxidize or reduce other substances. PPM (parts per million) is a concentration unit for a specific chemical in water (for example, free chlorine, dissolved oxygen, or another analyte). There is no universal, chemistry-independent conversion from ORP to ppm; at best, ORP can sometimes be empirically correlated to the ppm of a particular oxidant/disinfectant residual under fixed conditions. Higher ORP often indicates more oxidizing conditions, but it does not uniquely determine “contaminant ppm,” and the direction/magnitude of any correlation depends on the system.

How to Calculate Orp To Ppm?

The following steps outline how to estimate ppm from ORP using the simplified formula: PPM = ORP / K (where K is a calibration factor for your specific application).

  1. First, measure the ORP (oxidation-reduction potential) value (mV) with your ORP probe/meter.
  2. Next, determine the conversion factor (K) value (mV/ppm) from your own calibration data for the specific substance and conditions.
  3. Next, gather the formula from above: PPM = ORP / K.
  4. Finally, calculate the estimated concentration in parts per million (ppm).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge (assumes you have calibrated a linear relationship for your specific system).

ORP (oxidation-reduction potential) = 600 mV

Conversion factor (K) = 300 mV/ppm